The mechanisms of multiple myeloma (MM) initiation, evolution, and resistance/relapse are enigmatic. By defining common changes in the gene expression profile (GEP) of tumor cells and the microenvironment (ME), critical pathways involved in disease evolution may be identified. Project 3 will define molecular signatures of disease initiation, evolution, and resistance/relapse by studying the GEP of ME and MM and by examining molecular consequences of MM-ME interactions. Because the stromal cells of the bone ME may be transiently or permanently modified in MM and contribute to tumorigenesis, we will test the hypothesis that GEP of the ME component of the MM bone marrow will enable us to identify key molecular mechanisms through which the ME contributes to the disease phenotype. Two aims will rely on samples obtained through clinical research performed in Projects 1 and 2. Aim 1: Identify and validate candidate genes involved in pathogenetic crosstalk between the MM and ME that contribute to the MM phenotype. This will be accomplished by (a) identifying ME-associated GEP changes that are linked to tumor development and plasma cell dyscrasia, (b) correlating ME-associated genes (MAGs) with MM PC GEP-defined or disease characteristic-defined subgroups to identify potential tumor cell-ME crosstalk mechanisms, and (c) identifying common drug-specific qualitative or quantitative changes in the MM PC and ME GEP after short-term (48-hour) drug treatment. Aim 2: Develop and validate GEP as a prognostic indicator of clinical outcome to form a rational basis for selecting treatment in the future. Patient outcomes (complete response, event-free and overall survival) will be correlated with GEP of the MM and ME. Specifically, through Aim 2 we will (a) determine whether the GEP changes found in Aim 1c after short-term (48-hour) drug treatment correlate with clinical outcome; (b) assess whether baseline GEP signatures of ME, MM PCs, or both predict outcome; and (c) compare GEP of ME and MM PCs at baseline, remission, and relapse to identify molecular determinants of resistance and pathways associated with disease progression. The bone marrow ME is a complex heterogenous mixture of cells with each having a distinct GEP. However, with the large sample groups available through the P01, recurring patterns found in MM combined with validation studies will provide insight into the key molecular signals contributing to disease progression originate. Elucidating the key GEP signatures in MM disease evolution will provide clinically relevant information that can be used to develop targeted therapies and ultimately improve treatment.